Abstract: A structural stress based shear strength theory was established for transverse fillet welds in this paper, the new equations for calculating shear strength of fillet welds were provided, and some problems about shear strength of fillet welds were investigated. The results show that, theoretical failure angle of welds subjected unidirectional shear force is 22.5°, and both weld toe angle and load condition could also affect failure angle. In addition, the weld with 30° weld toe angle subjected unidirectional shear force possesses maximum load capacity, and the load capacity of weld in biaxial loading is higher than which in unidirectional loading. The theory in this paper was verified by test data, and theoretical bases were provided for weld sizing.